How can I kill the LAG?

[Eragon]

My problem is this: when i enter the game thee are no lags its perfect!! I start the game and its great! But when a dog appears everything starts getting slow! And when i'm being attacked its like seeing photos passing. When i hear the dog attacking it appears to me the dog on the ground and when i swing the axe it doesnt appear moving. Just looks like i'm seeing a photoshow with sound. This is what i call lag. Strange i have games with much better graphics and efects and they are not laggy and penumbra just gets realy slow!!!!!

When i fight on penumbra i'm seeing a photoshow, but when i kill the dog passed maybe 20min(of killing the dog), it goes back to normal.
I really need help on this

EDITED: This vid shows how i play penumbra with no monsters.(it lags more on the vid because of the sfotware that was recording.) SPOILER VID: http://www.youtube.com/watch?v=23HAldnFLjA